code: def get_open_spaces(board):
      """Given a representation of the board, returns a list of open spaces."""
      open_spaces = []
      for i in range(3):
          for j in range(3):
              if board[i][j] == 0:
                  open_spaces.append(encode_pos(i, j))
      return open_spaces
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import gym
  from gym import spaces
  from gym.utils import seeding
  import numpy
  from tensor2tensor.data_generators import problem
  from tensor2tensor.layers import modalities
  from tensor2tensor.rl import gym_utils
label_desc: Use image node_id 3 for calling a global function with example usage: get_open_spaces(board) and returns: open_spaces